This invention relates to the continuous manufacture of structural shapes, particularly from heat-hardenable resins that are reinforced with continuous strands, webs, or mats of mineral fibers, particularly glass fibers.
It is known to produce structural shapes from fiber reinforced resins by drawing a length of resin-impregnated mineral fiber material through a fluid- or electrically-heated die. The die raises the temperature of the material and causes the resin to polymerize. In these arrangements, thermal transfer is accomplished by conduction as the material engages the surfaces of the die. This results in the polymerization of resin located at or near the exterior surfaces of the shape before polymerization of the resin in the interior of the shape; this can result in inducing internal cracking. In order for sufficient heat to be transferred to the material being processed under these arrangements, the operation must take place at relatively low speeds or very long dies must be used; either of these solutions has an adverse effect on the economics of the operation.
Dielectric heating of fiber-reinforced thermosetting resins is also known. Dielectric heating substantially eliminates the problem of internal cracking because the temperature of the resin is raised to the polymerization initiation point simultaneously throughout the cross-section of the shape being formed. However, known systems require the use of nonmetallic dies that are formed of a material having low dielectric loss. Polytetrafluoroethylene (PTFE) is a preferred material for this purpose. However, PTFE dies have the disadvantage that the die surface is quickly worn away, and this changes the dimensions of the die and results in difficulties in holding the tolerances of the shape being produced.
It has also been proposed to manufacture certain shapes by successively forming the resin impregnated material with the aid of rollers, which rollers also form the electrodes of a dielectric heating arrangement for providing heat to accelerate the polymerization of the resin. However, this method is disadvantageous because the heating occurs in short bursts, the variety of shapes that can be manufactured is limited, and operating speeds are low.
A two-stage method is also known. In this method, at a first stage, dielectric heating initiates the polymerization of the resin and, simultaneously, the forming of the shape by means of a die is effected. In this case, the die is of a material having a low dielectric loss. During the second stage, a final polymerization is carried out as the material passes through a metallic die, not subject to the influence of the electric field of the dielectric heater, but heated by conventional means. In certain cases, this final polymerization is completed as the material passes through an oven. The shaping and final hardening that takes place in the second stage requires a relatively long metallic die; this creates processing difficulties arising from the relatively high coefficient of friction between the die and the material being shaped and heated. In addition, this method requires the use of expensive dies and uneconomical dual heating arrangements having a heated forming die and a heated finishing die.
This invention does away with the above-noted disadvantages.